Oblique projection optical system and projection type display apparatus using the same

ABSTRACT

A projection type display apparatus including an oblique projection optical system having a plurality of lenses is disclosed. A lens nearest to a projection screen has a vertical effective image area through which a light flux passes. The lens is arranged at a position not including an optical axis shared by the largest number of lenses among the plurality of lenses. A flat mirror for returning the optical path is arranged between the particular lens and the projection screen at a predetermined angle to the optical axis. An enlarged image obtained by the light flux returned by the flat mirror is formed toward a display screen.

BACKGROUND OF THE INVENTION

This invention relates to a projection type display apparatus forenlarging an image displayed on a display screen of a display device andprojecting the enlarged image on a projection screen or a projectionboard, and a projection optical system thereof, or in particular to aprojection type display apparatus for projecting obliquely on theprojection screen an image displayed on the display screen of a displaydevice, an optical system thereof, and a method of machining a moldedplastic lens constituting the optical system.

In the projection type display apparatus for displaying in an enlargedform, on a projection screen or a projection board, an image on adisplay screen of a liquid crystal panel of reflection type ortransmission type or a displayed image of a display device with an arrayof a plurality of micro mirrors arranged thereon, it is important notonly to produce a sufficiently enlarged image on the projection screenbut also to prevent the shadow of a presenter from entering theprojection screen and the enlarged image light from intruding directlyinto the eyes of the presenter. To meet this requirement, a projectionoptical system of what is called the short projection type in which thedistance between the projection type display apparatus and theprojection screen is decreased has begun to be placed on the market.This projection optical system is so configured that the enlarged imagelight enters the projection screen from an oblique direction (forexample, JP-A-2008-250296).

Means for optically adjusting the oblique projection optical systemusing a curved mirror for oblique projection is also known (for example,JP-A-2002-35077).

On the other hand, a projection type display apparatus of rearprojection type is known in which a mirror for returning optical path isinterposed between the projection type display apparatus and theprojection screen to decrease the apparent projection distance (forexample, JP-A-2006-259252).

SUMMARY OF THE INVENTION

In the conventional art, or especially in an oblique projection opticalsystem disclosed in JP-A-2008-250296 in which enlarged image lightenters a projection screen from an oblique direction, a constitution isadopted in which a curved mirror is arranged between the projectionoptical system and the projection screen, and an intermediate imageformed between the curved mirror and a coaxial projection optical systemis enlarged by the curved mirror and projected as an enlarged image onthe projection screen.

According to the technique disclosed in JP-A-2008-250296, therefore, thecurved mirror is required to be moved in parallel along the optical axisof the coaxial projection optical system to change the magnification ofthe enlarged image on the screen, and for this purpose, a high-accuracymovement adjusting mechanism to prevent the curved mirror from tiltingwith respect to the optical axis. Such an adjusting mechanism, however,is not disclosed in JP-A-2008-250296.

JP-A-2002-350774, on the other hand, discloses an adjusting method inwhich a free-form curved mirror is simply moved, and fails to take intoconsideration a specific method of correcting phenomena unique to theoblique projection optical system such as trapezoidal distortion of theprojected image due to the oblique projection on the projection screenand the aberration due to a difference in projection distances in thevertical direction on the projection screen. Also, JP-A-2002-350774describes nothing about a method of fabricating the free-form curvedmirror having negative power arranged between the projection opticalsystem and the projection screen.

Further, the projector disclosed in JP-A-2006-259252 includes a mirrormechanism to rotate a mirror for reflecting light projected from aprojection lens of the projector body, in which a fixing unit of themechanism is fixed on the projector body to maintain the projectionlight from the projector body at a predetermined projection angle to themirror. Thus, the rear projection is possible without shifting theprojection lens in the projector body on the one hand, and the projectorbody is accommodated in the mirror mechanism for front projection(direct projection on the screen) on the other hand. In this way, thisprojector is designed to have a compact form in the case where theprojector body is accommodated in the mirror.

In the projector disclosed in JP-A-2006-259252, however, the relativepositions of the mirror and the projector body for rear projection arefixed, and there is not consideration on a method of changing themagnification of the enlarged image on the screen and a technical meansfor adjusting the position of the enlarged image for rear projection.

The present invention has been achieved in view of the aforementionedproblems, and an object thereof is to provide an oblique projectionoptical system for projecting an image obliquely on the projectionscreen and a projection type display apparatus using such a system,wherein the trapezoidal distortion and the aberration due to the obliqueprojection are corrected with a compact configuration and in the casewhere the projection light is returned by a flat mirror and projected onthe projection screen, limitation on the setting of the projection typedisplay apparatus is remarkably improved.

Another object of the invention is to provide a projection type displayapparatus including an oblique projection optical system configured of aplurality of lenses including at least one aspheric plastic lens,wherein the plastic lens is formed in an aspheric shape and thereby themachining time of the molding die is reduced as compared with the lenshaving a free-form curved surface, and the employment of a flat mirrorfor returning the optical path remarkably reduces the development cost.

In order to achieve the objective described above, according to oneaspect of the invention, there is provided what is called an obliqueprojection optical system including a plurality of lenses for enlargingthe image displayed on a display screen and projecting the enlargedimage obliquely on a projection screen, wherein the lens nearest to theprojection screen has a vertical effective area of the image throughwhich an image light flux is passed and which is arranged at a positionnot containing the optical axis shared by a largest number of lensesamong the plurality of lenses, and wherein the shape of the lens isasymmetrical about the optical axis thereof, thereby making it possibleto correct the aberration caused by the ultra-wide angle and thedistortion caused by the oblique projection.

For the reason described above, in the oblique projection optical systemaccording to the invention, the height (shift amount) of the position ofthe displayed enlarged image with respect to the optical axis in thevertical direction of the screen can be increased, and therefore, asmall flat mirror for returning the optical path can be arranged betweenthe lens located at a nearest position to the projection screen and theprojection screen.

Also, in order to reduce the size of the projection type displayapparatus having the oblique projection optical system, an upper end ofthe effective area of the lens nearest to the projection screen in thevertical direction is arranged above an lower end of the effective areaof the flat mirror in the vertical direction of the screen, and whereinthis flat mirror has a predetermined angle of elevation with respect tothe optical axis shared by the largest number of the lenses of theoblique projection optical system as well as this angle of elevation ischangeable by a rotation adjusting mechanism.

Further, the oblique projection optical system according to theinvention can maintain the image quality even at an ultra-wide angle.Accordingly, in the projection type display apparatus having thisoblique projection optical system, a high magnification image can beproduced even in the case where the distance is short between theprojection screen and the projection type display apparatus, and theflat mirror can be moved by a flat mirror moving mechanism along theoptical axis shared by the largest number of lenses.

Furthermore, the lens constituting the oblique projection optical systemaccording to the invention and arranged nearest to the projection screenhas the vertical effective area of the image, through which the lightflux is passed, which is arranged at a position not containing theoptical axis shared by the largest number of lenses among the pluralityof lenses, and which has such a shape asymmetric about the center axisof the effective area thereof and a portion of the aspheric shapesymmetric about the optical axis shared by the largest number of lensesis truncated.

According to another aspect of the invention, there is provided aprojection type display apparatus including an oblique projectionoptical system configured of a plurality of lenses for enlarging theimage displayed on the display screen and projecting the enlarged imageobliquely on the projection screen, wherein the lens nearest to theprojection screen has a vertical effective area of the image, throughwhich the light flux passes, which is arranged at a position notincluding the optical axis shared by the largest number of lenses amongthe plurality of lenses, and which is accommodated in the range ofmaximum vertical width of the surface of the housing of the projectiontype display apparatus facing the projection screen.

According to still another aspect of the invention for achieving theobjective described above, there is provided a projection type displayapparatus including an oblique projection optical system configured of aplurality of lenses for enlarging the image on the display screen andprojecting the enlarged image obliquely on the projection screen,wherein the lens nearest to the projection screen has a verticaleffective area of the image through which the image light flux passesand which is arranged at a position not including the optical axisshared by the largest number lenses among the plurality of the lenses,wherein a flat mirror for returning the optical path is arranged betweenthe lens nearest to the projection screen and the projection screen andhas a rotation adjusting mechanism for changing the angle of the flatmirror with respect to the optical axis shared by the largest number oflenses, wherein in a first state where the flat mirror is arranged at apredetermined angle with respect to the optical axis shared by thelargest number of lenses, an image enlarged by the image light fluxreturned by the flat mirror is obtained toward the display screen, whilein a second state where the flat mirror is accommodated in theprojection type display apparatus, an enlarged image is obtained in thedirection along the extended optical axis shared by the largest numberof lenses among the plurality of lenses.

According to still another aspect of the invention, there is provided aprojection type display apparatus including a unit for detecting therotation angle of the flat mirror and an image correction function forautomatically correcting the distortion of the image projected on thescreen in accordance with the detected rotation angle.

According to yet another aspect of the invention, there is provided aprojection type display apparatus wherein in the case where the flatmirror for returning the optical path is arranged at a predeterminedangle θ1 with respect to the axis perpendicular to the optical axisshared by the largest number of lenses, an enlarged image is obtainedfrom the image light flux returned by the flat mirror as an imageenlarged in the direction toward the display screen, the projection typedisplay apparatus being tilted by θ2 with respect to a reference flatsurface substantially perpendicular to the enlarged image, where θ1 andθ2 satisfy the following relationship:1.5≦θ2/θ1≦2.0

According to a further aspect of the invention, there is provided anoblique projection optical system, wherein the lens nearest to theprojection screen among the plurality of lenses is formed of plasticsand has a vertical effective area of the image, through which the lightflux passes, which is arranged at a position not including the opticalaxis shared by the largest number of lenses among the plurality oflenses, and which has such a shape that a portion of the aspheric shapesymmetric about the optical axis shared by the largest number of thelenses constituting the projection optical system is truncated, therebymaking it possible to machine a plurality of molding dies arrangedsymmetrically about the optical axis shared by the largest number oflenses.

According to a still further aspect of the invention, there is providedan oblique projection optical system for projecting an image obliquelyto the projection screen and a projection type display apparatus usingthe oblique projection optical system, wherein the trapezoidaldistortion and the aberration due to the oblique projection arecorrected with a compact structure that a small flat mirror can beaccommodated in the projection type display apparatus, wherein in thecase where the projection light is returned by the flat mirror andprojected on the projection screen, the position of the projection imagecan be changed by adjusting the angle of the flat mirror, and whereinthe limitation on the setting of the projection type display apparatuscan be remarkably improved by moving the flat mirror in the directionalong the optical axis and thus changing the magnification of theprojection image.

According to a yet further aspect of the invention, there is provided anoblique projection optical system configured of a plurality of lenses,wherein the lens nearest to the projection screen has a verticaleffective area of the image, through which the image light flux passes,and which is arranged at a position not including the optical axisshared by the largest number of lenses among the plurality of lenses,and

wherein the particular lens has such a shape that a portion of theaspheric shape symmetric about the particular optical axis is truncatedto remarkably reduced the time for machining the molding dies, and aflat mirror is used for the mirror for returning the optical path for agreatly reduced development cost.

Other objects, features and advantages of the invention will becomeapparent from the following description of the embodiments of theinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a projection type display apparatus includingan oblique projection optical system and an optical path return mirroraccording to an embodiment of the invention.

FIG. 2 is a side view of a projection type display apparatus includingan oblique projection optical system and an optical path return mirroraccording to another embodiment of the invention.

FIG. 3 is a side view of a projection type display apparatus includingan oblique projection optical system and an optical path return mirroraccording to still another embodiment of the invention.

FIG. 4 is a side view of a projection type display apparatus includingan oblique projection optical system and an optical path return mirroraccording to yet another embodiment of the invention.

FIG. 5 is a side view of a projection type display apparatus includingan oblique projection optical system and an optical path return mirroraccording to a further embodiment of the invention.

FIG. 6 is a front view of a projection type display apparatus includingan oblique projection optical system and an optical path return mirroraccording to an embodiment of the invention.

FIG. 7 is a side view of a projection type display apparatus includingan oblique projection optical system according to an embodiment of theinvention.

FIG. 8 is a front view of a projection type display apparatus includingan oblique projection optical system according to an embodiment of theinvention.

FIG. 9 is a sectional view of the projection lens showing the lensconfiguration of the oblique projection optical system according to thisinvention.

FIG. 10 is a sectional view showing the lens configuration of theoblique projection optical system and the light ray tracking resultaccording to the invention.

FIG. 11 is a sectional view showing the lens configuration of theoblique projection optical system and the light ray tracking resultaccording to the invention.

FIG. 12 is a sectional view of the projection lens having the lensconfiguration of the oblique projection optical system according to anembodiment (the lens data shown in FIGS. 25A, 25B) of the invention.

FIG. 13 is a sectional view of the projection lens having the lensconfiguration of the oblique projection optical system according toanother embodiment (the lens data shown in FIGS. 26A, 26B) of theinvention.

FIG. 14 is a sectional view of the projection lens having the lensconfiguration of the oblique projection optical system according tostill another embodiment (the lens data shown in FIGS. 27A, 27B) of theinvention.

FIG. 15 is a sectional view illustrating the lens configuration of theoblique projection optical system and the light ray tracking resultaccording to an embodiment (the lens data shown in FIGS. 25A, 25B) ofthe invention.

FIG. 16 is a diagram representing the spot shape of the projection imageof the oblique projection optical system according to an embodiment (thelens data shown in FIGS. 25A, 25B) of the invention.

FIG. 17 is a diagram representing the spot shape of the projection imageof the oblique projection optical system according to another embodiment(the lens data shown in FIGS. 26A, 26B) of the invention.

FIG. 18 is a diagram representing the spot shape of the projection imageof the oblique projection optical system according to still anotherembodiment (the lens data shown in FIGS. 27A, 27B) of the invention.

FIG. 19 is a diagram illustrating a configuration of the illuminationoptical system of the projection type display apparatus according to anembodiment.

FIG. 20 is a diagram schematically illustrating the configuration of alens surface shaping machine for the die to mold the plastic lens.

FIG. 21 is a diagram schematically illustrating a method of themachining operation on the lens surface shaping machine for the die tomold the plastic lens.

FIG. 22 is a diagram schematically illustrating the method of shapingthe lens surface for the die to mold the plastic lens according to anembodiment of the invention.

FIG. 23 is a diagram schematically illustrating the method of shapingthe lens surface for the die to mold the plastic lens according toanother embodiment of the invention.

FIG. 24 is a diagram illustrating the contour of the plastic lensaccording to an embodiment of the invention.

FIG. 25A represents the data on the spherical system included in thelens data adapted for the projection lens to realize the obliqueprojection optical system according to a first embodiment of theinvention.

FIG. 25B represents the data on the aspheric system included in the lensdata adapted for the projection lens to realize the oblique projectionoptical system according to the first embodiment of the invention.

FIG. 26A represents the data on the spherical system included in thelens data adapted for the projection lens to realize the obliqueprojection optical system according to a second embodiment of theinvention.

FIG. 26B represents the data on the aspheric system included in the lensdata adapted for the projection lens to realize the oblique projectionoptical system according to the second embodiment of the invention.

FIG. 27A represents the data on the spherical system included in thelens data adapted for the projection lens to realize the obliqueprojection optical system according to a third embodiment of theinvention.

FIG. 27B represents the data on the aspheric system included in the lensdata adapted for the projection lens to realize the oblique projectionoptical system according to the third embodiment of the invention.

FIG. 28 is a sectional view of the projection lens illustrating the lensconfiguration of the oblique projection optical system according to anembodiment of the invention.

FIG. 29 is a diagram illustrating the configuration of a part of theprojection type display apparatus according to an embodiment of theinvention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The best aspect for implementing the invention is explained in detailbelow with reference to the accompanying drawings. Incidentally, in eachof the drawings, elements having a same function are designated by samereference numerals, respectively, and explanations thereof are omitted.

FIGS. 1 to 3 are side views each schematically illustrating a projectiontype display apparatus according to an embodiment of the invention.Especially, a projection type display apparatus is provided with anoblique projection optical system (not shown) in which a flat mirror(not shown) is fixed on a fixing frame 2 and an image light flux 3 isreturned and projected obliquely on a projection screen directed towarda display screen (not shown). The main parts including the obliqueprojection optical system described above, an illumination opticalsystem and circuit parts (not shown) are accommodated in a lower housing9 and an upper housing la.

In FIGS. 1 to 3, reference numeral 4 designates a flat mirrorrotation/fixing mechanism, and numeral 5 a rotation mechanism capable ofadjusting the angle of the mirror fixing frame and, if required,includes a detection unit capable of detecting the angle of elevation ofthe flat mirror is provided. Numerals 6 and 7 designate fixing units forfixing a flat mirror moving mechanism, numeral 8 the flat mirror movingmechanism, R1 an upper limit of the enlarged image light flux in thevertical direction of the screen, and R2 an lower limit of the enlargedimage light flux in the vertical direction of the screen.

Incidentally, a specific example of the lens configuration for realizingthe oblique projection optical system of the projection type displayapparatus according to this invention is explained in detail later.

In the projection type display apparatus according to this invention, asshown in FIG. 2, the flat mirror (not shown) is fixed on the fixingframe 2, and the image light flux 3 can be returned and projectedobliquely on the projection screen in the direction toward the displayscreen (not shown). Accordingly, not only the apparent projectiondistance from the projection type display apparatus to the projectionscreen can be greatly reduced, but also the flat mirror can be moved bythe moving mechanism 8 relatively to the housing of the projection typedisplay apparatus and fixed at a predetermined position by the fixingunits 6, 7, thereby making it possible to change the projection distanceto the projection screen without moving the housing. As a result, notonly the magnification of the image on the projection screen can beeasily changed, but also, as shown in FIGS. 3 and 4, the angle of theflat mirror can be adjusted from a predetermined angle of elevation bythe flat mirror rotation/fixing mechanism 4 or the mirror fixing framerotation mechanism 5, thereby making it possible to move the imagedisplay position on the projection screen arbitrarily.

At the same time, one or both of the flat mirror rotation/fixingmechanism 4 and the mirror fixing frame rotation mechanism 6 is providedwith a rotation angle detection unit (for example, a rotary encoder)capable of detecting the rotation angle of the flat mirror with respectto the predetermined angle of elevation, so that the automatic Keystonecorrection of the video circuit can be made in the vertical direction ofthe screen in accordance with the rotation angle obtained thereby tofurther improve the facility in operation.

Also, the projection type display apparatus according to this invention,as shown in FIG. 2, is designed in such a manner that in the case wherethe flat mirror (not shown) is fixed on the fixing frame 2 and the imagelight flux 3 is returned and projected obliquely on the projectionscreen in the direction toward the display screen (not shown), in orderto reduce the effective area of the flat mirror, the interval betweenthe projection type display apparatus and the flat mirror is shortened,and the amount of shift (defined as 10:0 in the case where the opticalaxis of the projection lens is superposed on the lower vertical end ofthe enlarged image, and as a negative value in the case where the lowervertical end of the enlarged image is above the optical axis assumingthat the enlarged image width in the vertical direction is 10) of theoblique projection optical system assumes a negative value (i.e. so thatthe lower vertical end of the enlarged image is located above theoptical axis).

In the projection type display apparatus according to this invention, inorder to further shorten the interval between the projection typedisplay apparatus and the flat mirror, the flat mirror is tilted, asshown in FIG. 4, by θ1 from a predetermined angle of elevation by therotation/fixing mechanism 4 or the mirror fixing frame rotationmechanism 5, and the projection type display apparatus is tiltedcorrespondingly by θ2 from the plane perpendicular to the projectionscreen. In this way, an enlarged image having little distortion of theprojection screen can be obtained. In connection with this, theprojection type display apparatus according to this invention wasactually manufactured by way of trial and the relationship between thevertical keystone distortion correction and the image quality reductiondue to the image processing was studied.

As a result, it was found that in the case where the ratio between θ2and θ1 (θ2/θ1) is not larger than 1.5, the magnification of the image inthe upper part of the projection screen in the vertical direction wasenlarged than that of the image in the lower part thereof to such anexcessive degree that the image quality is deteriorated even after thevertical Keystone correction was executed in the video circuit. On thecontrary, in the case where the ratio between θ2 and θ1 (θ2/θ1) is notless than 2.5, the magnification of the image in the lower part of theprojection screen in the vertical direction was enlarged as comparedwith that of the image in the upper part thereof to such an excessivedegree that the image is deteriorated considerably even after making thevertical Keystone correction was executed by the video circuit. It hasthus been found that in the case where the ratio between θ2 and θ1(θ2/θ1) is set to near 2.0, the image quality deterioration can beminimized.

FIGS. 5 and 6 are a side view and a front view, respectively,schematically illustrating the projection type display apparatusaccording to this invention in which the flat mirror (not shown) forreturning the optical path is accommodated in the upper part of the sethousing.

In the projection type display apparatus according to this invention, asillustrated in FIG. 5, the flat mirror (not shown) for returning theoptical path fixed to the fixing frame 2 can be accommodated at apredetermined position in the upper part of the set housing by the flatmirror rotation/fixing mechanism 4, the mirror fixing frame rotationmechanism 5, the moving mechanism 8 and the fixing units 6, 7, and whenthe flat mirror is accommodated at such a position, the enlargedprojection is possible in the direction along the optical axis shared bythe largest number of lenses constituting the oblique projection opticalsystem without returning the optical path.

FIG. 6 is a front view of the set schematically illustrating theprojection type display apparatus according to this invention when theflat mirror (not shown) for returning the optical path is accommodatedin the upper part of the set housing. Among the plurality of lensesconstituting the oblique projection optical system, the lens (designatedby L17 in FIG. 6) arranged at a position nearest to the projectionscreen is made to be a rectangle or a trapezoid shape havingsubstantially the same aspect ratio as the effective area of the displayscreen. The vertical effective image area of this lens, which shuts offunrequired light and through which the light flux passes, is arranged ata position not including the optical axis shared by the largest numberof lenses among the plurality of lenses, thereby the vertical effectiveimage area of the lens is accommodated within the maximum verticalscreen width of the surface of the housing of the projection typedisplay apparatus facing the projection screen, resulting in aremarkably improved design quality.

The structure of the projection type display apparatus according to anembodiment of this invention has been described above in which anenlarged image is obtained regardless of whether the flat mirror forreturning the optical path is accommodated or set open as shown in FIGS.5 and 6. Nevertheless, a projection type display apparatus which is notusable when the optical path returning flat mirror is accommodated inthe housing is of course within the scope of the present invention, aslong as being provided with the oblique projection optical systemaccording to this invention.

The projection type display apparatus according to another embodiment ofthis invention, as shown in FIG. 7, having no flat mirror for returningthe optical path, is configured to project the image in enlarged form inthe direction along the optical axis shared by the largest number oflenses constituting the oblique projection optical system.

FIG. 8 is a front view of the set schematically illustrating a form ofthe projection type display apparatus according to another embodiment ofthis invention shown in FIG. 7, in which the lens (designated as L17 inFIG. 8) nearest to the projection screen among the plurality of lensesconstituting the oblique projection optical system is of a rectangle ora trapezoid shape having substantially the same aspect ratio as theeffective display screen area thereof, and the vertical effective imagearea of the lens to shut off the unrequired light and permit light fluxto pass therethrough is arranged at a position not containing theoptical axis shared by the largest number of lenses among the pluralityof lenses. The vertical effective image area of the particular lens isthus accommodated within the maximum vertical screen width of thesurface of the housing of the projection type display apparatus facingto the projection screen. Further, the center of the contour of the lensL17 mentioned above is located above the center line of the surface ofthe housing facing to the projection screen, thereby to exhibit asatisfactory balance of the appearance and a remarkably improved designquality.

Next, with reference to FIGS. 9 to 18, an oblique projection opticalsystem employed in the projection type display apparatus is specificallyexplained. In order to minimize the projection distance of theprojection type display apparatus, the lens nearest to the projectionscreen among the plurality of lenses constituting the oblique projectionoptical system is of a rectangle or a trapezoid shape havingsubstantially the same aspect ratio as the effective display screen areathereof, and the vertical effective image area of the lens to shut offthe unrequired light and permit the light flux to pass therethrough isarranged at a position not containing the optical axis shared by thelargest number of lenses among the plurality of lenses. In this way, ashort projection distance for oblique projection in enlarged form issecured.

<Oblique Projection Optical system for Short Distance Projection>

First, FIG. 9 is a sectional view illustrating the basic configurationof the projection optical system illustrated by the YZ section of theXYZ orthogonal coordinate system. For the convenience of explanation ofthe projection optical system, the liquid crystal panel 122 providingthe display screen and the cross prism 111 are shown on the right sideand the projection screen on the left side. This embodiment correspondsto the lens data represented in FIGS. 27A, 27B, and the lens L17arranged at a position nearest to the projection screen is an asphericplastic lens, of which the effective area for passing the image lightflux is arranged at a position not including the optical axis 11 sharedby a plurality of lenses constituting the oblique projection opticalsystem. Thus, the light flux focused around the projection screen can becontrolled simply by the shape of the lens L17, thereby making itpossible to correct the trapezoidal distortion due to the obliqueprojection or the aberration due to the ultra-wide angle lens(especially, a high-order coma aberration and astigmatism). Also, theshape of the lens L17 is a rectangle having the aspect ratiosubstantially equal to that of the effective area of the display screenor a trapezoid corresponding to the area where the image light flux ispassed, resulting in an advantage of shutting off the unrequired lightwhich otherwise might deteriorate the focusing performance. Further, thecontour of the lens L17 which is not circular symmetric about theoptical axis 11 makes it possible to reduce the size. As a result, evenin the case where the oblique projection optical system according tothis embodiment is accommodated in the housing of the projection typedisplay apparatus, the contour of the lens L17 can be accommodatedwithin the maximum vertical screen width of the surface of the housingfacing to the projection screen. At the same time, the center of thecontour of the lens L17 is located above the center line of the surfaceof the housing facing the projection screen. Thus, a satisfactorybalance of the external appearance can be secured and at the same timethe design quality can be remarkably improved.

In FIG. 9, only the lens L17 is formed in the shape of a rectanglehaving substantially the same aspect ratio as the effective area of thedisplay screen or a trapezoid (in the figure, the sectional shape isshown) corresponding to the area through which the image light fluxpasses. In the lens configuration for realizing the oblique projectionoptical system according to this invention, however, as illustrated inFIG. 15, the contours of the lenses L14 and L15, which also have an areanot permitting the image light flux to pass therethrough, may bedetermined excepting such an area. In this way, the contour size can bereduced as compared with the contour of the conventional lens symmetricwith respect of the optical axis, and the projection type displayapparatus having this optical system can be effectively reduced in sizeand weight.

Although the lenses L3 and L11 are also aspheric plastic lenses, theeffective area of each lens, through which the image light flux passes,is arranged at a position including the optical axis 11 shared by aplurality of lenses constituting the oblique projection optical system,and therefore, has an aspheric shape symmetric about the optical axis11. The projection lens for realizing the oblique projection opticalsystem according to this embodiment is composed of totally 17 lensesincluding 14 glass lenses and 3 plastic lenses and fixedly held by fourlens barrels (B1, B2, B3, B4). Incidentally, in the case where themagnification is changed by changing the projection distance, the focuscan be adjusted by changing the position of the lens barrel B4 relativeto the position of the lens barrel B3.

In the embodiment shown in FIGS. 10 to 15, the origin of the XYZorthogonal coordinate system is set at the center of the display screenof the liquid crystal panel 122 for displaying an image by modulatingthe illumination light flux with video signals, and the Z axis isassumed to be parallel to the normal to the liquid crystal panel 122(not shown) for displaying the image. The Y axis is parallel to a shortside of the display screen of the liquid crystal panel 122 fordisplaying the image, and assumed to be equal to the vertical directionof the image display liquid crystal panel 122. The X axis is parallel toa long side of the display screen of the liquid crystal panel 122 fordisplaying the image, and assumed to be equal to the horizontal(lateral) direction of the liquid crystal panel 122 for displaying theimage. Also, FIG. 10 is a sectional view of the projection lens of theoblique projection optical system of the projection type displayapparatus according to an embodiment, and FIG. 10 is a sectional view ofthe projection lens including but not showing the flat mirror forreturning the optical path.

A sectional view of the projection lens for realizing the obliqueprojection optical system according to a first embodiment of thisinvention is shown in FIG. 12, data on a spherical system included inlens data obtainable in the process is represented in FIG. 25A, and dataon an aspheric system similarly obtainable is represented in FIG. 25B.Also, a sectional view of the projection lens for realizing the obliqueprojection optical system according to a second embodiment is shown inFIG. 13, data on a spherical system included in lens data obtainable inthe process is represented in FIG. 26A, and data on an aspheric systemobtainable is represented in FIG. 26B. Similarly, a sectional view ofthe projection lens for realizing the oblique projection optical systemaccording to a third embodiment is shown in FIG. 14, data on a sphericalsystem included lens data obtainable in the process is represented inFIG. 27A, and the data on an aspheric system similarly obtainable isrepresented in FIG. 27B.

In each lens data, projection distances L0, L1, an amount of shift S1from the optical axis at the image display position and a vertical imagesize Dv represented in FIGS. 10 and 11 are summarized in Table 1 below.

TABLE 1 L0(mm) Dv(mm) S1(mm) 60″ Projection Embodiment 1 650.1 747.1186.8 60″ Projection Embodiment 2 651.3 747.1 186.8 60″ ProjectionEmbodiment 3 650.0 747.1 186.8 80″ Projection Embodiment 1 885.6 996.1249.0 80″ Projection Embodiment 2 885.6 996.1 249.0 80″ ProjectionEmbodiment 3 882.4 996.1 249.0 100″  Projection Embodiment 1 1011.71245.0 331.3 100″  Projection Embodiment 2 1011.9 1245.0 331.3 100″ Projection Embodiment 3 1011.8 1245.0 331.3

In the projection lens for realizing the oblique projection opticalsystem according to the invention, the amount of shift S1 for the sizeDv in the vertical direction of the image can be set to 20% or more.Also, the ratio D/L0 between the projection screen size D (mm) and theprojection distance L0 (mm) for the general the projection type displayapparatus of 60 inches has a screen size of 1524 (mm) and a projectiondistance of 1800 (mm), and in this case the ratio D/L0 is 0.85. Ashorter projection distance is about 1000 (mm), and in this case theratio D/L0 is 1.52. In the oblique projection optical system accordingto this invention, as described above, even a D/L of 2.0 or more isrealizable, and according to an embodiment, a ratio of 2.34 is actuallyobtained.

According to this invention, the projection type display apparatushaving a flat mirror for returning the optical path can also be realizedas shown in FIG. 11. The result of manufactured by way of trialindicates that the interval between the lens arranged at a positionnearest to the projection screen and the flat mirror is about 150 mm,and therefore, L1 represented in FIG. 11 is shortened by about 150 mmcompared to L0. Also, in the case where the depth of the set is assumedto be 350 mm, the distance from the projection type display apparatusaccording to the invention to the projection screen with the lightreturned by the flat mirror is shortened by 500 mm compared to L0. Thisleads to great advantages that a large image can be obtained with asmall installation space and the presenter does not directly view theimage light of the projection type display apparatus.

Next, a specific method of reading the lens data and the mechanism forcorrecting the aberration of the projection lens to realize the obliqueprojection optical system according to an embodiment of the inventionare explained with reference to the lens data represented in FIGS. 25A,25B for the projection lens according to the first embodiment having aconfiguration in FIG. 12. In general, the lenses are configured in threegroups, and those L16 to L12 from the projection screen side form thethird group. All of L16 to L13 are concave lenses having a telocentricconfiguration. At the same time, in order to reduce the chromaticaberration of magnification, a convex lens formed of a lens materialsmall in Abbe number is arranged as L12. Further, as represented in FIG.15, L16 is arranged at a position not including the optical axis 11shared by a plurality of lenses constituting the oblique projectionoptical system. In this way, a strong aspheric shape in which the lightflux focused around the screen can be controlled by the lens shape ofL16 alone used to correct the trapezoidal distortion caused by theoblique projection and the aberration due to the ultra-wide angle lens(especially, the high-order coma aberration and the astigmatism). In theprocess, the contour of the lens L16 is not required to be circularsymmetric about the optical axis for the aforementioned reason, but maybe a rectangle shape having an aspect ratio substantially equal to thatof the effective area of the displays screen or a trapezoidcorresponding to the area through which the image light flux is passed.As a result, unrequired light which otherwise might deteriorate thefocusing performance can be advantageously shut off. Further, the factthat the contour of the lens L16 is not circular symmetric about theoptical axis can reduce the size, with the result that even in the casewhere the oblique projection optical system according to this embodimentis accommodated in the housing of the projection type display apparatus,the contour of L16 can be set within the vertical maximum screen widthof the surface of the housing facing to the projection screen. At thesame time, the contour center of L16 is located above the center line ofthe surface of the housing facing the projection screen, resulting in asatisfactory balance of external appearance for a remarkably improveddesign property.

Further, L11 to L9 make up a second group, in which L11 is a lens havinga strong aspheric shape with a weak negative refractive power, andcorrects the spherical aberration and the low-order coma aberrationcaused by a light flux substantially parallel to the optical axis andpassing through a place distant from the optical axis. The lenses L10and L9, on the other hand, take charge of a part of the refractive powerof the glass projection lens having a positive refractive power, and atthe same time the lens L9 suppresses the occurrence of the comaaberration and the astigmatism in the shape of a meniscus convex towardthe projection screen.

Finally, L8 to L1 constitute a first group, in which the doublet lens ofL8 and L7 and the triplet lens of L6 to L4 are rendered to have anegative refractive power and thus a strong telecentric property.Further, the lens L3 having a strong aspheric shape corrects zonal comaaberration caused by a light flux oblique to the optical axis andpassing through a place distant from the optical axis. As a result, thedistortion was suppressed and a satisfactory focusing performance of theprojection lens as a whole was realized in oblique projection. Theprojection lenses according to other embodiments of the inventionillustrated in FIGS. 13, 14 are equivalent to a configuration in whichL15 illustrated in FIG. 12 is divided into L15 and L16 for an improvedaberration correction ability and in which the aspheric lens L16illustrated in FIG. 12 is simply replaced by the lens L17, while theresult that the effects and the configuration of the projection opticalsystem remain the same.

Next, the oblique projection optical system described above is furtherexplained using specific numerical values with reference to FIGS. 25A,25B, FIGS. 26A, 26B and FIGS. 27A, 27B.

First, FIG. 12 shows the configuration of the projection optical systemaccording to this embodiment based on an example of the numerical valueslisted in FIGS. 25A, 25B. In other words, FIG. 12 shows theconfiguration of the YZ section in the XYZ orthogonal coordinatedescribed above. The image projection apparatus according to theinvention, as illustrated in FIGS. 1 to 6, is realizable also in aconfiguration having a mirror for returning the optical path (notshown). For convenience' sake, however, an explanation is given on theassumption that the mirror for returning the optical path is not presentas illustrated in FIGS. 7, 8. The configuration of the projectionoptical system of FIG. 12 is shown as a development in the directionalong Z axis. This is also the case with FIGS. 13 to 15.

Light emitted from the display screen P0 (a liquid crystal panelaccording to the embodiment) indicated under the optical axis 11 in FIG.12 passes first through the first and second groups configured of onlythe projection lenses having a rotationally symmetric surface among theplurality of lenses. Then, the image light passes through the thirdgroup including the aspheric lens L16 rotationally asymmetric about thecenter of the lens contour and projected in enlarged form on theprojection screen.

Here, the first and second groups of the projection lenses areconfigured of a plurality of lenses all having rotationally symmetricrefraction surfaces, and four of the refraction surfaces arerotationally symmetric aspheric surfaces, while the other surfaces arespherical. The rotationally symmetric surfaces here are expressed byEquation (1) represented in FIG. 25B using the local cylindricalcoordinate system for each surface.

In Equation (1), h is the distance from the optical axis, Z is the sagamount of the lens surface shape, c is the radius of curvature at thevertex, K is the conic constant and A to J are the coefficients of theterms of the powers of h.

Also, the radius of curvature of each surface is listed in FIG. 25A. InFIG. 25A, the radius of curvature having its center on a left side ofthe surface is expressed by a positive value, and otherwise, by anegative value. In FIG. 25A, the inter-surface distance indicates thedistance from the vertex of a lens surface to the vertex of a next lenssurface. The inter-surface distance is expressed as a positive value inthe case where the next lens surface is located on the left side of agiven lens surface in FIG. 25A, and as a negative value in the casewhere the next lens surface is located on the right side. Further, inFIG. 25A, the surface numbers (9), (10), (23), (24), (33) and (34)indicate aspheric surfaces rotationally symmetric about the opticalaxis, “aspheric”being written beside the surface number in the table ofFIG. 25A to be easily understood.

The coefficients of these six aspheric surfaces are listed in FIG. 25B.

From the table represented in FIG. 25B, it is understood that the conicconstant K is 0 according to this embodiment. The trapezoidal distortioncaused by the oblique incidence is very large in the direction ofoblique incidence, and small in the direction perpendicular to thedirection. Therefore, considerably different functions are required inthe direction of oblique incidence and in the direction perpendicularthereto. Thus, the asymmetric aberration can be satisfactorily correctedby not using the conic constant K functioning in all the directions inthe rotational symmetry.

Incidentally, the numerical values described in the tables of FIGS. 25A,25B assume a case in which an optically modulated optical image(modulated image) in a range of 0.59 inch diagonal with an aspect ratioof 16×9 on the display screen of the liquid crystal panel is projectedin a form enlarged to sizes of 60 inches, 80 inches and 100 inchesdiagonal on the projection screen. In order to secure the optimumfocusing performance for the enlarged image of each size, the lenses L15and L16 are moved in parallel to the optical axis in such a manner thatthe lens intervals (30) and (34) assume the inter-surface intervalslisted in the lower table in FIG. 25B.

The lens data listed in FIGS. 26A, 26B and 27A, 27B are also written ina similar format.

Spot shapes of the image projected in the form enlarged to 80 inchesusing the lens data numerically listed in FIGS. 25A, 25B according tothe first embodiment are represented in FIG. 16, and spot shapes of theimage projected in the form enlarged to 80 inches using the lens datanumerically listed in FIGS. 26A, 26B according to the second embodimentare represented in FIG. 17. Further, spot shapes of the image projectedin the form enlarged to 80 inches using the lens data numerically listedin FIGS. 27A, 27B according to the third embodiment are represented inFIG. 18.

In FIG. 16, the spots of light fluxes emitted from ten points includingthe X-Y locating at (0, 3.67), (−6.53, 3.67), (−3.92, 2.20), (0.0, 0.0),(0, 0), (−6.53, 0.0), (−3.92, −2.20), (0, −3.67), (−5.53, −3.67),(−5.22, −3.67), (−5.22, 3.67) on the display screen of the liquidcrystal panel are represented in the order from the bottom to up. Itsscale unit is 5 mm. The horizontal direction in the spot diagramcorresponds to the X direction on the display screen of the liquidcrystal panel, and the vertical direction corresponds to the Y directionon the display screen of the liquid crystal panel. The spot diagramsrepresented in FIGS. 17, 18 are also obtained by the light flux from thepoints of similar values on the X-Y coordinate on the display screen ofthe liquid crystal panel. From these diagrams, it is understood that asatisfactory performance is maintained.

An oblique projection optical system for realizing the projection typedisplay apparatus according to an embodiment of the invention have beendescribed in detail above. The example described above is so configuredthat the light emitted from the projection lens is returned by theoptical path returning flat mirror and proceeds toward the displayscreen of the liquid crystal panel. Nevertheless, the invention is notlimited to this configuration, and depending on the position where theprojection lens is arranged, the returning flat mirror may of course beomitted.

Next, an illumination optical system used in the projection type displayapparatus according to an embodiment of the invention is explained withreference to FIG. 19. In FIG. 19, a light source 101 includes a lamp 98and a reflector 99. The lamp 98 is a high-pressure white mercury lamp.Also, the reflector 99, arranged in such a position as to cover the lamp98 from behind, has a reflection surface in a shape of a paraboloid ofrevolution and a circular or polygonal exit opening. Light emitted fromthis lamp 98 is reflected by the reflector 99 having a reflectionsurface in the shape of the paraboloid of revolution, proceeds in adirection substantially parallel to an optical axis 115, and is emittedfrom the light source 101 as a substantially parallel light flux. Thelight emitted from the light source 101 enters an integrator ofmulti-lens type 103.

As described above, the multi-lens integrator 103 is configured of afirst multi-lens element 103 a and a second multi-lens element 103 b.The first multi-lens element 103 a is configured of a matrix array of aplurality of lens cells each in a shape of a rectangle substantiallysimilar to the liquid crystal panels 122 a, 122 b, 122 c as viewed fromthe direction along the optical axis 115. The light incident from thelight source is divided into a plurality of light rays through theplurality of the lens cells and thereby efficiently led through thesecond multi-lens element 103 b and a polarization conversion element104. Specifically, the first multi-lens element 103 a is designed sothat the lamp 98 and each lens cell of the second multi-lens element 103b are in optically conjugate with each other.

The plurality of lens cells of the second multi-lens element 103 b, likethose of the first multi-lens element 103 a, are each in a shape of arectangle as viewed from the direction along the optical axis 115 andarranged in matrix. Each of the lens cells constituting the secondmulti-lens element 103 b projects (maps) the shape of the correspondinglens cell of the first multi lens element 103 a on the liquid crystalpanels 122 a, 122 b and 122 c in cooperation with the superpositionlenses 108 a, 108 b and 108 c. In the process, the polarizationconversion element 104 functions to set the light from the secondmulti-lens element 103 b in a predetermined direction of polarization.At the same time, the image projected through each lens cell of thefirst multi-lens element 103 a is superposed by the superposition lenses108 a, 108 b and 108 c, thereby the light amount is uniformlydistributed on the corresponding liquid crystal panels 112 a, 112 b and112 c.

Among the plurality of lenses constituting the oblique projectionoptical system according to the invention, the lens arranged nearest tothe projection screen is made of plastics, and the effective areathereof in the vertical direction of the image through which the lightflux passes is arranged at a position not containing the optical axisshared by the largest number of lenses among the plurality of lenses.This plastic lens is in such a shape that a portion of the asphericshape symmetric about the optical axis shared by the largest number oflenses constituting the projection optical system is truncated.

The molding die is machined by a method shown in FIG. 21(A) in which adie being a work is rotated and cut with a cutting tool on amulti-spindle machine to produce a die in the shape corresponding to thedesired lens shape, or by a method shown in FIG. 21(B) in which a workis fixed while rotating a cutting tool thereby to machine the die to therequired shape. To obtain a mirror surface, the method (B) requires thecutting time about 10 to 20 times longer than the method (A). The method(B) is advantageously used to machine a rotationally asymmetricfree-form curved surface, while the method (A) is suitable to machininga rotationally symmetric aspheric work in a shorter time.

Among the plurality of the lenses constituting the oblique projectionoptical system according to the invention, the lens arranged nearest tothe projection screen is made of plastics, and the vertical effectiveimage area thereof through which the light flux passes is arranged at aposition not containing the optical axis shared by the largest number oflenses among the plurality of lenses. This plastic lens is in such ashape that a portion of the aspheric shape symmetric about the opticalaxis shared by the largest number of lenses constituting the projectionoptical system is truncated. Thus, the method of machining the moldingdie described above can be employed in which as shown in FIG. 21(A), thedie being a work is rotated and cut with the cutting tool on themulti-spindle machine tool thereby to produce a die in the shapecorresponding to the desired lens shape. As a result, the die machiningoperation is made possible in a short machining time and the developmentcost can be reduced.

In the process, according to this invention, a plurality of dies can bemachined at the same time with the C axis as a rotation axis of themulti-spindle machine tool shown in FIG. 21(A). In the case where twodies are machined at the same time, for example, as shown in FIG. 22,two dies (work pieces) are arranged symmetrically about the rotationaxis. Then, the desired die shape can be obtained with a high accuracyin satisfactory cutting balance in the machining operation. Further,FIG. 23 illustrates an optimum arrangement for machining four dies atthe same time. Also in the case where an odd number of dies are machinedat the same time, a similar effect can of course be obtained byarranging the adjacent dies at positions apart from each other by therotation angle (360 degrees) divided by the number of the dies machinedat the same time.

In the oblique projection optical system according to the inventiondescribed above, the contour shape of the lens arranged at a positionnearest to the projection screen is, for example, a rectangle as shownin FIG. 24. In that case, the shape of the lens surface is asymmetricabout the center axis based on the contour shape of the effective lenssurface, but an aspheric shape symmetric about the optical axis sharedby the largest number of lenses constituting the oblique projectionoptical system.

Incidentally, in FIG. 9, only the lens L17 nearest to the projectionscreen is formed in a shape lacking the area where the image light fluxis not passed, and arranged at a position not including the optical axis11 shared by the largest number of lenses of the oblique projectionoptical system. Nevertheless, not only the lens L17 arranged nearest tothe projection screen but also a plurality of lenses next nearest to theprojection screen may be formed in the shape lacking the area where theimage light flux is not passed. In this way, the oblique projectionoptical system can be reduced in size and weight.

Specifically, in the configuration illustrated in FIG. 28, for example,not only the lens L17 nearest to the projection screen but also the lensL16 next nearest to the projection screen is arranged at a position notcontaining the optical axis 11 shared by the largest number of lenses ofthe oblique projection optical system. Further, the lens L15 arranged atthe second next nearest position to the projection screen has a contourin a shape lacking a lower end area through which the image light fluxis not passed. In the configuration shown in FIG. 28, therefore, ascompared with the shape having the area through which the image lightflux is not passed, the structure of the oblique projection opticalsystem can be reduced in size and weight by an amount equivalent to aspace 12 defined by a dotted line.

In the configuration shown in FIG. 28, the lens L17 is formed ofplastics and arranged at a position not containing the optical axisshared by the largest number of lenses constituting the projectionoptical system. This plastic lens has such a shape that a portion of theaspheric shape symmetric about the optical axis is truncated. Thus, themolding die explained with reference to FIG. 20 can be machined in ashorter time at a lower initial cost, and resultingly, the lens can befabricated at a low cost. Further, the lens L16, though formed of glass,is arranged at a position not containing the optical axis shared by thelargest number of lenses constituting the projection optical system, andhas such a shape that a portion of the spherical shape symmetric aboutthe optical axis is truncated. Two lenses L16 can be fabricated bycutting one spherical glass into two parts, thereby making it possibleto fabricate even glass lens at a low cost. Specifically, the greaterthe number of lenses which are arranged at a position not containing theoptical axis 11 shared by the largest number of lenses constituting theoblique projection optical system and which have such a shape that aportion of the aspheric or spherical shape symmetric about the opticalaxis is truncated, the less expensively the oblique projection opticalsystem can be fabricated. In the configuration shown in FIG. 28, onlythe lenses L17 and L16 correspond to such lenses. Nevertheless, astructure is desirable in which not only the single lens nearest to theprojection screen, but as many lenses as possible not less than two,arranged nearest to the projection screen and having area through whichthe image light flux is not passed, are arranged at a position notcontaining the optical axis 11 shared by the largest number of lenses ofthe oblique projection optical system, and formed in such a shape that aportion of the aspheric or spherical shape symmetric about the opticalaxis is truncated.

As a result, like in the configuration of the oblique projection opticalsystem illustrated in FIG. 28, not only the lens nearest to theprojection screen but also a predetermined number of lenses successivelyarranged from the position nearest to the projection screen are formedin the shape lacking the area through which the image light flux is notpassed. In this way, the oblique projection optical system can bereduced in both size and weight and can be fabricated at a lower cost.

Thus, the use of this oblique projection optical system more preferablymakes it possible to fabricate a compact, lightweight and low costprojection type display apparatus.

With the configuration of the oblique projection optical systemillustrated in FIG. 28, the size, weight and cost can be reduced forboth the projection type display apparatus having the flat mirror asshown in FIGS. 1 to 6 and the projection type display apparatus havingno flat mirror as illustrated in FIGS. 7 and 8.

With reference to FIG. 29, an explanation is given about an example inwhich the size of the projection type display apparatus having a flatmirror is suitably reduced by utilizing the space 12 of the obliqueprojection optical system thereof illustrated in FIG. 28.

FIG. 29 illustrates a portion of the oblique projection optical systemof the projection type display apparatus having a flat mirror, a flatmirror 14, a mirror holding unit 15 for holding the flat mirror, and amirror rotation shaft 13 providing a support to rotate the mirrorholding unit 15. In the example shown in FIG. 29, the mirror rotationshaft 13 is arranged in a space corresponding to the space 12 shown inFIG. 28. In FIG. 29, the flat mirror is shown at a position forprojecting an image, and when the flat mirror is accommodated in theupper part of the set housing, it can be accommodated by rotating themirror holding unit 15 around the mirror rotation shaft 13 in theillustrated direction.

In the projection type display apparatus having the flat mirrorillustrated in FIGS. 1 to 6, the flat mirror is accommodated in theupper part of the set housing by the flat mirror rotation/fixingmechanism 4, the rotation mechanism 5 and the moving mechanism 8 for themirror fixing frame, the fixing unit 6 and the fixing unit 7. Namely,the position of the flat mirror is changed by the two types ofrotational motions and the two types of slide motions.

On the contrary, in the configuration illustrated in FIG. 29, only bythe rotational motion around the mirror rotation shaft 13, the anglethat the flat mirror forms to the optical axis 11 shared by the largestnumber of lenses of the oblique projection optical system can be changedthereby to change the operating and accommodation of the positions ofthe flat mirror. Specifically, in the configuration illustrated in FIG.29, both the operating and the accommodation positions of the mirror canbe changed by a simpler rotation adjusting mechanism including themirror rotation shaft 13 and the mirror holding unit 15.

Further, in the example illustrated in FIG. 29, the mirror rotationshaft 13 is formed of a single metal shaft to improve the holdingaccuracy of the mirror rotation shaft 13. If it is supposed that asingle mirror rotation shaft formed of metal is attempt to be used in adisplay apparatus not having the compact structure of the obliqueprojection optical system as illustrated in FIG. 28, the position of themirror rotation shaft would interfere with the lenses of the obliqueprojection optical system unless the position of the mirror rotationshaft is located ahead in the direction of projection or lower than theposition corresponding to the space 12 illustrated in FIG. 28. In theexample illustrated in FIG. 29, in contrast, the mirror rotation shaft13, which is arranged in a space corresponding to the space 12illustrated in FIG. 28, is not required to be arranged at a positionahead in the direction of projection or lower than the positioncorresponding to the space 12, even if it is formed of a single metalshaft, thereby preventing the set housing from becoming bulky.

Specifically, the configuration illustrated in FIG. 29 can furtherreduce the size of the projection type display apparatus in a suitableway.

In FIG. 29, the mirror rotation shaft 13 is arranged at a positioncorresponding to the space 12. FIG. 28 illustrates that the space 12includes the space formed continuously from the position nearest to theprojection screen, at a position not including the optical axis 11shared by the largest number of lenses of the oblique projection opticalsystem and being inverted symmetrically about the optical axis 11 withrespect to the space which is occupied by the plurality of lenses whichhave such a shape that a portion of the aspheric or spherical shapesymmetric about the optical axis is truncated.

Accordingly, if the mirror rotation shaft 13 is arranged at a positionsuperposed on at least a portion of the space inverted symmetricallyabout the optical axis 11 with respect to the space which is occupied bythe plurality of lenses formed continuously from the position nearest tothe projection screen at a position not including the optical axis 11shared by the largest number of lenses of the oblique projection opticalsystem and which have such a shape that a portion of the aspheric orspherical shape symmetric about the optical axis is truncated, then atleast the effect similar to that of the configuration shown in FIG. 29can be obtained.

It should be further understood by those skilled in the art thatalthough the foregoing description has been made on embodiments of theinvention, the invention is not limited thereto and various changes andmodifications may be made without departing from the spirit of theinvention and the scope of the appended claims.

The invention claimed is:
 1. A projection type display apparatuscomprising a group of lenses having a plurality of lenses which enlargesand projects an image of a display device on a projection screen,wherein said group of lenses has a first lens arranged at a nearestposition to said projection screen, wherein a flat mirror which reflectsan optical path is arranged between said first lens and said projectionscreen, wherein an effective area of an image light flux which passesthrough said first lens does not include an optical axis shared by aplurality of lenses which do not include at least said first lens amongsaid group of lenses, wherein said flat mirror is arranged at apredetermined angle with respect to said optical axis, and theprojection type display apparatus further comprising a moving mechanismor moving said flat mirror of the oblique projection optical systemalong said optical axis.
 2. A projection type display apparatuscomprising a group of lenses having a plurality of lenses which enlargesand projects an image of a display device on a projection screen,wherein said group of lenses has a first lens arranged at a nearestposition to said projection screen, wherein a flat mirror which reflectsan optical path is arranged between said first lens and said projectionscreen, wherein an effective area of an image light flux which passesthrough said first lens does not include an optical axis shared by aplurality of lenses which do not include at least said first lens amongsaid group of lenses, wherein said flat mirror is arranged at apredetermined angle with respect to said optical axis, wherein an upperend of a vertical effective area of said first lens is located above alower end of a vertical effective area of said flat mirror, and whereinthe apparatus further comprises a moving mechanism or moving said flatmirror of the oblique projection optical system along said optical axis.3. A projection type display apparatus comprising a group of lenseshaving a plurality of lenses which enlarges and projects an image of adisplay device on a projection screen, wherein said group of lenses hasa first lens arranged at a nearest position to said projection screen,wherein a flat mirror which reflects an optical path is arranged betweensaid first lens and said projection screen, wherein an effective area ofan image light flux which passes through said first lens does notinclude an optical axis shared by a plurality of lenses which do notinclude at least said first lens among said group of lenses, whereinsaid flat mirror is arranged at a predetermined angle with respect tosaid optical axis, and the projection type display apparatus furthercomprising: a rotation adjusting mechanism adapted to change an angle ofsaid flat mirror of said oblique projection optical system with respectto said optical axis; means for detecting said rotation angle of saidflat mirror; and an image correction function for automaticallycorrecting distortion of a projected image in accordance with saiddetected rotation angle.
 4. The projection type display apparatusaccording to claim 1, comprising: a rotation adjusting mechanism adaptedto change an angle of said flat mirror of said oblique projectionoptical system with respect to said optical; means for detecting saidrotation angle of said flat mirror; and an image correction function forautomatically correcting distortion of a projected image in accordancewith said detected rotation angle.
 5. A projection type displayapparatus comprising a group of lenses having a plurality of lenseswhich enlarges and projects an image of a display device on a projectionscreen, wherein said group of lenses has a first lens arranged at anearest position to said projection screen, wherein a flat mirror whichreflects an optical path is arranged between said first lens and saidprojection screen, wherein an effective area of an image light fluxwhich passes through said first lens does not include an optical axisshared by a plurality of lenses which do not include at least said firstlens among said group of lenses, wherein said flat mirror is arranged ata predetermined angle with respect to said optical axis, and wherein anupper end of a vertical effective area of said first lens is locatedabove a lower end of a vertical effective area of said flat mirror, theapparatus further comprising: a rotation adjusting mechanism adapted tochange an angle of said flat mirror of said oblique projection opticalsystem with respect to said optical axis; means for detecting saidrotation angle of said flat mirror; and an image correction function forautomatically correcting distortion of a projected image in accordancewith said detected rotation angle.
 6. A projection type displayapparatus comprising a group of lenses having a plurality of lenseswhich enlarges and projects an image of a display device on a projectionscreen, wherein said group of lenses has a first lens arranged at anearest position to said projection screen, wherein a flat mirror whichreflects an optical path is arranged between said first lens and saidprojection screen, wherein an effective area of an image light fluxwhich passes through said first lens does not include an optical axisshared by a plurality of lenses which do not include at least said firstlens among said group of lenses, wherein said flat mirror is arranged ata predetermined angle with respect to said optical axis, and wherein incase where said flat mirror is arranged at a predetermined angle θ1 withrespect to said optical axis, an enlarged image obtained by an imagelight flux reflected by said flat mirror is directed toward a displayscreen of said display device, said projection type display apparatus isarranged being tilted by θ2 with respect to a reference flat surfacesubstantially perpendicular to said enlarged image, and said angles θ1and θ2 satisfy a relationship,1.5≦θ2/θ1≦2.5.
 7. A projection type display apparatus comprising a groupof lenses having a plurality of lenses which enlarges and projects animage of a display device on a projection screen, wherein said group oflenses has a first lens arranged at a nearest position to saidprojection screen, wherein a flat mirror which reflects an optical pathis arranged between said first lens and said projection screen, whereinan effective area of an image light flux which passes through said firstlens does not include an optical axis shared by a plurality of lenseswhich do not include at least said first lens among said group oflenses, wherein said flat mirror is arranged at a predetermined anglewith respect to said optical axis, wherein among said plurality oflenses constituting said oblique projection optical system, apredetermined number of successive lenses from said position nearest tosaid projection screen are all arranged at positions not including saidoptical axis, and wherein said predetermined number of said successivelenses have such a shape that a portion of aspheric or spherical shapesymmetric about said optical axis is truncated.
 8. The projection typedisplay apparatus according to claim 6, wherein among said plurality oflenses constituting said oblique projection optical system, apredetermined number of successive lenses from said position nearest tosaid projection screen are all arranged at positions not including saidoptical axis, and wherein said predetermined number of said successivelenses have such a shape that a portion of aspheric or spherical shapesymmetric about said optical axis is truncated.
 9. A projection typedisplay apparatus comprising a group of lenses having a plurality oflenses which enlarges and projects an image of a display device on aprojection screen, wherein said group of lenses has a first lensarranged at a nearest position to said projection screen, wherein a flatmirror which reflects an optical path is arranged between said firstlens and said projection screen, wherein an effective area of an imagelight flux which passes through said first lens does not include anoptical axis shared by a plurality of lenses which do not include atleast said first lens among said group of lenses, wherein said flatmirror is arranged at a predetermined angle with respect to said opticalaxis, wherein among said plurality of lenses constituting said obliqueprojection optical system, a predetermined number of successive lensesfrom said position nearest to said projection screen are all arranged atpositions not including said optical axis, and wherein saidpredetermined number of said successive lenses have such a shape that aportion of aspheric or spherical shape symmetric about said optical axisis truncated, said projection type display apparatus comprising: a flatminor arranged between said lens nearest to said projection screen andsaid projection screen; and a rotation adjusting mechanism adapted tochange an angle of said flat mirror with respect to said optical axis;wherein said rotation adjusting mechanism includes a mirror holding unitfor holding said flat mirror, and a mirror rotation shaft providing acenter around which said mirror holding unit is rotated, and whereinsaid mirror rotation shaft is arranged in at least partially superposedrelation with a space which is an inversion of a space occupied by saidpredetermined number of said successive lenses from said positionnearest to said projection screen, about said optical axis.
 10. Theprojection type display apparatus according to claim 6, wherein amongsaid plurality of lenses constituting said oblique projection opticalsystem, a predetermined number of successive lenses from said positionnearest to said projection screen are all arranged at positions notincluding said optical axis, and said projection type display apparatuscomprising: a flat mirror arranged between said lens nearest to saidprojection screen and said projection wherein said predetermined numberof said successive lenses have such a shape that a portion of asphericor spherical shape symmetric about said optical axis is truncated,screen; and a rotation adjusting mechanism adapted to change an angle ofsaid flat mirror with respect to said optical axis; wherein saidrotation adjusting mechanism includes a mirror holding unit for holdingsaid flat mirror, and a mirror rotation shaft providing a center aroundwhich said mirror holding unit is rotated, and wherein said mirrorrotation shaft is arranged in at least partially superposed relationwith a space which is an inversion of a space occupied by saidpredetermined number of said successive lenses from said positionnearest to said projection screen, about said optical axis.